How is a primary alcohol oxidised to an aldehyde rather than a carboxylic acid?

The alcohol is heated gently with a limited amount of oxidising agent. The aldehyde has a low boiling point, so distilling it off as it forms prevents further oxidation.

Write an equation for the oxidation of ethanol to ethanal.

The equation is CH₃CH₂OH + [O] → CH₃CHO + H₂O. The symbol [O] represents oxygen from the oxidising agent.

How is a primary alcohol fully oxidised to a carboxylic acid?

The alcohol is heated under reflux with excess acidified potassium dichromate(VI). The equation for ethanol is CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.

CCEA A-Level Life and Health Sciences 0008

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Organic Chemistry

Oxidation of alcohols, IR spectroscopy and mass spectrometry

3 things to know
A2 2

Overview

Dehydration of alcohols, oxidation with acidified dichromate to aldehydes, acids and ketones, Benedict's and Fehling's tests, infrared absorptions and the fingerprint region, and mass spectra.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Write an equation for the dehydration of ethanol by concentrated phosphoric acid.

Key term

Which alkenes form when butan-2-ol is dehydrated?

Key term

What oxidising agent is used to oxidise alcohols, and what colour change shows that a reaction has occurred?

Questions from this subtopic

Each one with the model answer underneath.

1
Write an equation for the dehydration of ethanol by concentrated phosphoric acid.
Model answer
The equation is CH₃CH₂OH → CH₂=CH₂ + H₂O. The hot acid removes water in an elimination reaction.
2
Which alkenes form when butan-2-ol is dehydrated?
Model answer
Butan-2-ol gives a mixture of but-1-ene and but-2-ene. The but-2-ene forms as both E and Z isomers.
3
What oxidising agent is used to oxidise alcohols, and what colour change shows that a reaction has occurred?
Model answer
Potassium dichromate(VI) acidified with dilute sulfuric acid is used. The orange dichromate(VI) ions are reduced to green chromium(III) ions.
4
How is a primary alcohol oxidised to an aldehyde rather than a carboxylic acid?
Model answer
The alcohol is heated gently with a limited amount of oxidising agent. The aldehyde has a low boiling point, so distilling it off as it forms prevents further oxidation.
5
Write an equation for the oxidation of ethanol to ethanal.
Model answer
The equation is CH₃CH₂OH + [O] → CH₃CHO + H₂O. The symbol [O] represents oxygen from the oxidising agent.
6
How is a primary alcohol fully oxidised to a carboxylic acid?
Model answer
The alcohol is heated under reflux with excess acidified potassium dichromate(VI). The equation for ethanol is CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.

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